At the most basic level, slip rings comprise a conducting ring and a conducting brush. The ring and the brush are commonly incorporated onto rotor and stator structures to facilitate the intended rotational motion between the contacts. Slip rings are often integral to, or systematically incorporated with, electro-dynamic machines including heteropolar or homopolar electric motors and dynamos. In an electric motor or dynamo application, the slip ring provides electric current conduction to and from the machine's rotor as required to induce electromotive force to cause rotation of the rotor. Slip rings are also commonly used to provide electrical current to rotating machinery based systems, wherein the rotating element necessarily requires electrical power during rotation, i.e. rotating tanks, rotating weapon systems, rotating power shovels, etc. Whether the slip ring is used in a motor/generator or in a track type application, the current carrying requirements vary depending on use, and those requirements then drive a variety of slip ring designs.
With respect to the current invention, slip ring designs can generally be broken down into two groupings; low-current slip rings and high-current slip rings. Low-current slip rings are considered slip rings that can transmit milli-Amp current levels up to hundreds or even thousands of Amps. Low-current slip rings often have the ability to transfer current bi-directionally—i.e. clockwise and counter-clockwise rotation—while passing current, and are typically used for data and low power transfer. The brush elements that conduct the current are typically made from graphite blocks or cantilevered metallic “spring fingers” made from precious metals. The bi-directional capability is derived from a slip ring design comprising a cantilevered “spring finger” that lies across the ring tangentially as opposed to terminating at the ring. This configuration allows ring rotation in both directions due to appropriate lead-in angle present in both directions.
As the increase in current requires, available brush designs become limited. Because contact surface area requirements must meet these current requirements, single point or single line contact surface areas do not suffice for higher current requirements. The low current bi-directional slip ring incorporating tangential contact presents either point or line contact depending on the cross section of the spring finger. Accordingly, while allowing bi-directionality, this configuration limits the current transfer capability due to its limited contact geometry.
To accommodate higher current loads, brush geometry requires higher contact surface area than can be provided by the aforementioned tangential contact. A common approach is to align a plurality of “spring fingers” in a bank array, each of which terminates at the ring. These “spring fingers” are beam structures that are oriented perpendicular to the axis of rotation, and therefore parallel to the direction of surface motion, but are biased at an angle to the surface of the rotor or stator, as opposed to terminating orthogonally to the axis of rotation. The acute angle created between the “spring finger” and the surface of the rotor or stator is the lead-in angle. This brush element configuration allows for contact surface area that would be physically or economically infeasible for the aforementioned tangential contact arrangement. However, this configuration loses its inherent bi-directional capability because the terminal-end designs logically have only one lead-in angle as opposed to two.
Specifically, high-current brush elements are often comprised of either laminated metallic foil rings that are louvered in an array formation or individual “spring fingers” stamped from chosen brush material and bound in an array formation. The tips of louvers or the chevron stamping then terminate at the ring. High-current slip rings, configured in this manner, can transmit from hundreds of thousands to millions of Amps, but enable only one direction of motion while passing current. For this reason, high-current slip rings have primarily been used in experimental homopolar motors and generators. There is limited data on what happens when the rotor rotates opposite the lay of the brush elements—i.e. in the direction not having a lead-in angle—but it is assumed this would result in excessive friction, wear, and binding as the system more closely resembles a locked ratchet clutch.
To attain bi-directionality, a common solution has been to eliminate the lead-in angle and arrange the brush elements such that the beam section of each “spring finger” is in a parallel plane to the axis of rotation, which is then perpendicular to the direction of rotor or stator surface motion. In other words, the brush elements are arranged such that they have no bias towards motion and therefore have no lead-in angle. This configuration produces a brush element in which each “spring finger” is subject to cantilever beam forces and accompanying motion in both directions. Such bidirectional forces and motion decrease the life of the brush elements due to increased fatigue stress. Further, friction forces may spike when a change of direction takes place because the “spring fingers” must shift from accommodating motion in one direction to accommodating motion in the other. Thus, there is a prevalent lack of technology in the slip ring industry adequately addressing both mega-amp current requirements as well as bi-directionality.